1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave filter using a resonator and a coupler such as a multistrip coupler, directional coupler, or other suitable coupler. More specifically, the present invention relates to a surface acoustic wave device having low loss and a satisfactory shape factor, and a communication apparatus incorporating the same.
2. Description of the Related Art
Conventionally, a surface acoustic wave filter is known as an IF filter used in a mobile communication apparatus. Such a surface acoustic wave filter includes a known dual-mode surface acoustic wave resonator filter having a longitudinal coupling and a lateral coupling. In the double-mode surface acoustic wave resonator filter, because the pass bandwidth of the filter is significantly affected by an electromechanical coupling coefficient of a piezoelectric substrate defining the surface acoustic wave filter, it is impossible to substantially change the pass bandwidth of the filter by the changing the arrangement and structure of electrodes used in an IDT (interdigital transducer) and a reflector.
Recently, attention has been focused on surface acoustic wave filters, in which multistrip couplers are incorporated to change the pass bandwidths of the filters by changing the arrangement and structure of electrodes used in IDTs and reflectors, in addition to electromechanical coupling coefficients.
FIG. 11 is a plan view showing a surface acoustic wave filter 101 using a multistrip coupler.
As shown in FIG. 11, a first IDT 103, a second IDT 104, a multistrip coupler 105 and reflectors 103a, 103b, 104a, and 104b are provided on a piezoelectric substrate 102.
One of the comb-shaped electrodes of the first IDT 103 is connected to an input terminal 106, and the other one thereof is grounded. One of the comb-shaped electrodes of the second IDT 104 is connected to an output terminal 107, and the other one thereof is grounded.
The multistrip coupler 105 includes a plurality of strip lines, and is located on the right side in FIG. 11 so as to cover the area from the first IDT 103 to the second IDT 104.
The reflectors 103a and 103b are disposed in a surface-wave propagating direction in such a manner that the first IDT 103 and the multistrip coupler 105 are positioned between the reflectors 103a and 103b. The reflectors 104a and 104b are disposed in a surface-wave propagating direction in such a manner that the second IDT 104 and the multistrip coupler 105 are positioned between the reflectors 104a and 104b. 
The reflectors of the surface acoustic wave filter described above have specified frequency characteristics. Therefore, when it is necessary to adjust the frequency characteristics of the reflectors to the frequency characteristics of the IDTs, a number of reflectors are required. For example, in the case of frequencies from 100 to 300 MHz, approximately 80 reflectors are necessary.
In addition, in the case of the surface acoustic wave filter described above, because the number of the reflectors is increased and the lengths of the reflectors are thereby increased, the deviation of group-delay time characteristics increases.
In other words, as shown in FIG. 12, there is a tendency for both ends of group-delay time characteristics D in the pass band of the filter frequency characteristics C to increase, the surface acoustic wave signal detected at an output electrode delays such that the deviation of the characteristics D is approximately 0.5 xcexcS.
In addition, as the deviation of the group-delay time characteristics increases, the phase of a signal substantially changes. As a result, no proper response with respect to an input signal can be obtained. Therefore, in a communication apparatus incorporating the surface acoustic wave device, for example, in a communication apparatus such a cellular phone, even though a radio wave is received, there is a possibility that voice is cut off or no voice is output while talking over the phone.
Furthermore, since the lengths of the reflectors are increased, the dimensions of the overall surface acoustic wave device, such as a surface acoustic wave filter, are also increased. This is prevents miniaturization of the surface acoustic wave device.
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device in which the deviation of group-delay time characteristics is reduced and miniaturization of the device is facilitated.
A surface acoustic wave device according to a preferred embodiment of the present invention includes a piezoelectric substrate, a first interdigital transducer, a second interdigital transducer and a first coupler.
The first and second interdigital transducers are arranged on the surface of the piezoelectric substrate such that the second interdigital transducer is offset from a direction in which a surface acoustic wave excited by the first IDT propagates. A first edge is provided on the piezoelectric substrate to reflect the excited surface acoustic wave to the first and second interdigital transducers, and the first coupler having a plurality of metal strips on the piezoelectric substrate is provided between the first edge of the piezoelectric substrate and at least one of the first and second interdigital transducers so as to be adjacent to the first and second interdigital transducers. The surface acoustic wave device operates using a shear horizontal surface wave.
A surface acoustic wave device according another preferred embodiment of the present invention further includes a second coupler on the piezoelectric substrate arranged such that the first and second couplers interpose the first and second interdigital transducers.
A surface acoustic wave device according to yet another preferred embodiment of the present invention further includes a third interdigital transducer between the first coupler and the second coupler.
A surface acoustic wave device according to still another preferred embodiment of the present invention further includes a second edge on the piezoelectric substrate. The second edge is located on the side of at least one of the first and second interdigital transducers where first coupler is not provided, and the second edge is not perpendicular to the surface acoustic wave propagating direction.
The surface acoustic wave device further includes a resin film covering the first and second interdigital transducers.
The surface acoustic wave device may be incorporated in a communication apparatus or other electronic device.
According to preferred embodiments of the present invention, because the coupler is used to reflect a surface acoustic wave on a reflection end surface, group-delay time characteristics are flat, and the deviation of the group-delay time characteristics is thereby small. In addition, because loss occurring at a reflecting portion on the reflection end surface is suppressed more than that at a reflector, insertion loss is substantially reduced. Furthermore, since the reflecting portion on the reflection end surface is used instead of the reflector, miniaturization of the device is greatly facilitated.